Method of manufacturing a hysteretic MEMS two-dimensional thermal device

ABSTRACT

A MEMS hysteretic thermal device may be formed having two passive beam segments driven by a current-carrying loop coupled to the surface of a substrate. The first beam segment is configured to move in a direction having a component perpendicular to the substrate surface, whereas the second beam segment is configured to move in a direction having a component parallel to the substrate surface. By providing this two-dimensional motion, a single MEMS hysteretic thermal device may by used to close a switch having at least one stationary contact affixed to the substrate surface.

CROSS REFERENCE TO RELATED APPLICATIONS

This U.S. Patent Application is a divisional application of U.S. patentapplication Ser. No. 11/705,738 filed Feb. 14, 2007, issued as U.S. Pat.No. 7,626,311, which is a continuation-in-part of U.S. patentapplication Ser. No. 11/334,438, filed Jan. 19, 2006, issued as U.S.Pat. No. 7,548,145, each of which is hereby incorporated by reference inits entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

Not applicable.

STATEMENT REGARDING MICROFICHE APPENDIX

Not applicable.

BACKGROUND

This invention relates to a microelectromechanical systems (MEMS)thermal device, and its method of manufacture. More particularly, thisinvention relates to a MEMS thermal actuator which is constructed withat least two segments, each segment pivoting about a different point,and with motion hysteresis between the heating phase and the coolingphase.

Microelectromechanical systems (MEMS) are very small moveable structuresmade on a substrate using lithographic processing techniques, such asthose used to manufacture semiconductor devices. MEMS devices may bemoveable actuators, valves, pistons, or switches, for example, withcharacteristic dimensions of a few microns to hundreds of microns. Amoveable MEMS switch, for example, may be used to connect one or moreinput terminals to one or more output terminals, all microfabricated ona substrate. The actuation means for the moveable switch may be thermal,piezoelectric, electrostatic, or magnetic, for example.

FIG. 1 shows an example of a prior art thermal switch, such as thatdescribed in U.S. Patent Application Publication 2004/0211178 A1. Thethermal switch 10 includes two cantilevers, 100 and 200. Each cantilever100 and 200 contains a passive beam 110 and 210, respectively, whichpivot about fixed anchor points 155 and 255, respectively. A conductivecircuit 120 and 220, is coupled to each passive beam 110 and 210 by aplurality of dielectric tethers 150 and 250, respectively. When avoltage is applied between terminals 130 and 140, a current is driventhrough conductive circuit 120. The Joule heating generated by thecurrent causes the circuit 120 to expand relative to the unheatedpassive beam 110. Since the circuit is coupled to the passive beam 110by the dielectric tether 150, the expanding conductive circuit drivesthe passive beam in the upward direction 165.

Applying a voltage between terminals 230 and 240 causes heat to begenerated in circuit 220, which drives passive beam 210 in the direction265 shown in FIG. 1. Therefore, one beam 100 moves in direction 165 andthe other beam 200 moves in direction 265. These movements may be usedto open and close a set of contacts located on contact flanges 170 and270, each in turn located on tip members 160 and 260, respectively. Thesequence of movement of contact flanges 170 and 270 on tip members 160and 260 of switch 10 is shown in FIGS. 2 a-2 d, to close and open theelectrical switch 10.

To begin the closing sequence, in FIG. 2 a, tip member 160 and contactflange 170 are moved about 10 μm in the direction 165 by the applicationof a voltage between terminals 130 and 140. In FIG. 2 b, tip member 260and contact flange 270 are moved about 17 μm in the direction 265 byapplication of a voltage between terminals 230 and 240. This distance isrequired to move twice the 5 μm width of the contacts, a 4 μm initialoffset between the contact flanges 170 and 270, and additional marginfor tolerances of 3 μm. In FIG. 2 c, tip member 160 and contact flange170 are brought back to their initial position by removing the voltagebetween terminals 130 and 140. This stops current from flowing and coolsthe cantilever 100 and it returns to its original position. In FIG. 2 d,tip member 260 and contact flange 270 are brought back to nearly theiroriginal position by removing the voltage between terminals 230 and 240.However, in this position, tip member 160 and contact flange 170 preventtip member 260 and contact flange 270 from moving completely back totheir original positions, because of the mechanical interference betweencontact flanges 170 and 270. In this position, contact between the facesof contact flanges 170 and 270 provides an electrical connection betweencantilevers 100 and 200, such that in FIG. 2 d, the electrical switch isclosed. Opening the electrical switch is accomplished by reversing themovements in the steps shown in FIGS. 2 a-2 d.

SUMMARY

The switch construction and method of manufacture may be simplified if asingle MEMS actuator is capable of moving in two different directions,rather than having two MEMS actuators each moving in a single directionas shown. If a MEMS actuator is capable of moving in two differentdirections, then a MEMS switch using a fixed contact may be made using asingle MEMS actuator. Furthermore, if the motion of the device ishysteretic, i.e. the motion is different upon heating than it is uponcooling, the actuator may be designed so as to latch in a détenteposition against the contact. If such an actuator can be designed, thenthe control of the switch may also be simplified, because only thesingle actuator may need to be controlled. Accordingly, it is desirableto design and fabricate a MEMS actuator which is capable of moving intwo substantially different directions, and with motion which ishysteretic.

A MEMS device is described, which includes a cantilevered beam thatbends about one or more points in at least two substantially differentdirections. The MEMS device also includes a driving means coupled to thecantilever, wherein the driving means may include a drive beam tetheredto the cantilever by at least one tether. Upon heating the drive beam,the drive beam expands to deform the cantilever. Upon cooling the drivebeam, a heat sink located near the anchor point causes the drive beam tocool with a different temperature profile than it did upon heating, andtherefore the cantilever deflects along a different trajectory uponcooling than it did upon heating.

Embodiments of the MEMS device are described, which include a MEMSthermal actuator that may extend in two orthogonal directions by havingat least two segments disposed orthogonally to each other. Each segmentbends about a different point. Therefore, the MEMS hysteretic thermalactuator may have articulated motion, and be capable of moving in twosubstantially different directions.

Furthermore, the MEMS segmented thermal actuator may move along onetrajectory while heating up, but may move in a second, substantiallydifferent trajectory while cooling down. In other words, the motion ofthe segmented thermal actuator may be hysteretic during the heatingphase compared to the cooling phase. The segmented, hysteretic thermalactuator may therefore be used to close and latch an electrical switch,for example, as well as in any of a number of different applications,such as valves or pistons, which may require articulated, hystereticmotion.

Several embodiments of the MEMS segmented, hysteretic thermal actuatorare disclosed. In a first embodiment, two substantially differentdirections of motion are achieved by including a substantiallyninety-degree bend between two segments of a cool beam of the thermalactuator. A current-carrying element provides a hot driving beam, whichexpands relative to the cool beam. The current-carrying element isdisposed adjacent to the two segments of the cool beam and heats up ascurrent is driven through it. The current-carrying element expands uponheating, driving the first segment of the cool beam in one directionbefore the substantially ninety-degree bend, and driving the secondsegment of the cool beam in another direction after the substantiallyninety-degree bend. Because the temperature profile of the beam dependson whether the beam is being heated or cooled, the beam movesdifferently upon heating than it does upon cooling, and therefore themotion is hysteretic.

In another exemplary embodiment, the MEMS segmented, hysteretic deviceconsists of two segments and a rigid link joining the first segment tothe second segment in an approximately rectilinear fashion. Upon heatingan adjacent hot driving beam, the hot driving beam bends the firstsegment about its anchor point. Upon further heating, the hot drivingbeam bends the second segment about the rigid link. Upon cooling, thebending of the first segment about the anchor point relaxes before thesecond segment about the rigid link. Therefore, the motion of the MEMSsegmented actuator is hysteretic, being different upon heating than uponcooling.

In yet another exemplary embodiment, the segments of the MEMS segmented,hysteretic device are oriented to move in two substantially differentplanes. A flexure joins the two segments. A driving beam is a circuitwhich is disposed adjacent to the segments, such that the driving beamdrives the device in two different planes of motion, one about theanchor point and the other about the flexure.

These and other features and advantages are described in, or areapparent from, the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

Various exemplary details are described with reference to theaccompanying drawings, which however, should not be taken to limit theinvention to the specific embodiments shown but are for explanation andunderstanding only.

FIG. 1 is a schematic view of a prior art MEMS thermal switch;

FIGS. 2 a-2 d are diagrams illustrating the sequence of movementsrequired to close the switch illustrated in FIG. 1;

FIG. 3 is a diagram illustrating a first exemplary embodiment of a MEMSsegmented thermal actuator;

FIG. 4 is a plot of the results of a simulation predicting the behaviorof the MEMS hysteretic thermal actuator of FIG. 3 in response to theapplication of a square wave current pulse;

FIG. 5 is a plot of the results of a simulation predicting the behaviorof the MEMS hysteretic thermal actuator of FIG. 3 in response to theapplication of a first square wave current pulse followed by anothershort current pulse;

FIG. 6 is a diagram illustrating a second exemplary embodiment of a MEMShysteretic thermal actuator;

FIG. 7 is a diagram illustrating a third exemplary embodiment of a MEMShysteretic thermal actuator, in which a heater element is disposedadjacent to the drive beams;

FIG. 8 is a diagram illustrating a fourth exemplary embodiment of a MEMShysteretic thermal actuator, in which current flows through and heats ahigh coefficient of thermal expansion material along with a lowcoefficient of thermal expansion material;

FIG. 9 is a diagram illustrating a fifth exemplary embodiment of a MEMShysteretic thermal actuator, in which a heater is disposed adjacent to alow coefficient of thermal expansion member and a high coefficient ofthermal expansion member;

FIG. 10 is a diagram illustrating a sixth exemplary embodiment of a MEMShysteretic thermal actuator, in which a heat source is disposed at thebase of a heat conductor which conducts the heat to the tip of thethermal actuator;

FIG. 11 is a diagram illustrating a seventh exemplary embodiment of aMEMS hysteretic thermal actuator, in which a first segment is coupled toa second segment by a rectilinear rigid link;

FIG. 12 is a side view of an eighth exemplary MEMS hysteretic thermalactuator, wherein the motion of a first segment is in one plane, and themotion of a second segment is in another, substantially orthogonalplane;

FIG. 13 is a perspective rendering of the MEMS hysteretic thermalactuator of FIG. 12;

FIG. 14 is plan view of a ninth exemplary MEMS hysteretic thermalactuator, which moves in two substantially orthogonal planes;

FIG. 15 is a perspective view of the ninth exemplary embodimentillustrated in FIG. 14;

FIG. 16-19 illustrate the operation of the ninth exemplary MEMShysteretic thermal actuator; and

FIGS. 20-27 illustrate steps in an exemplary fabrication method for theninth exemplary MEMS hysteretic actuator.

DETAILED DESCRIPTION

Although the systems and methods described herein are applied to anelectrical switch, it should be understood that this is only oneembodiment, and that the systems and methods may be appropriate for anynumber of devices, such as valves, pistons and other devices usingactuators.

A MEMS hysteretic device is described, which includes a cantileveredbeam that extends from an anchor point in at least two orthogonaldirections. The MEMS hysteretic device also includes a driving meanscoupled to the cantilever, wherein the driving means may include a drivebeam coupled to the cantilever by at least one tether. Upon heating thedrive beam, the drive beam expands to deform the cantilever. Uponcooling the drive beam, a heat sink located near the anchor point causesthe drive beam to cool with a different temperature profile than it didupon heating, and therefore the cantilever deflects along a differenttrajectory upon cooling than it did upon heating.

FIG. 3 is a diagram illustrating a first exemplary embodiment of a MEMShysteretic thermal actuator 500. MEMS hysteretic thermal actuator 500includes two beam segments 300 and 400, which are joined at asubstantially ninety-degree joint 460. The first segment 300 includes afirst drive beam portion 320 disposed adjacent, and coupled to a firstcool, passive beam portion 310. Similarly, the second segment 400includes a second drive beam portion 420 disposed adjacent, and coupledto a second cool, passive beam portion 410. Current is input to thedrive beam portions 320 and 420 at contacts 330 and 340, and the currentcirculating in the drive beam circuit heats portions 320 and 420 byjoule heating. The drive beams 320 and 420 are mechanically coupled tothe passive beam portions 310 and 410 by dielectric tethers 350 and 450,respectively. The dielectric tethers 350 and 450 may be made of anyconvenient, non-conducting material which couples the drive beamportions 320 and 420 to segmented passive beam portions 310 and 410mechanically, but not electrically. In one embodiment, dielectrictethers 350 and 450 may be made from an epoxy-based photoresist such asSU-8, a negative photoresist developed by IBM of Armonk, N.Y.

The heat generated in the drive beam circuit flows out predominantlythrough the contacts 330 and 340, and to a lesser extent by radiationand convection to the closely spaced substrate, about 4 um from thedrive beam circuit. Because heat is generated all along the drive beams320 and 420, and flows out predominantly through the contacts 330 and340 which act as heat sinks, the point in the drive beam circuit whichis at the maximum temperature starts out being located adjacent to theninety degree joint 460 or at a location approximately midway to thedistal end of the drive beam 420. As the temperature continues to rise,however, the location of maximum temperature begins to move out alongthe drive beam circuit, away from contacts 330 and 340. If the durationof the current pulse is long enough, the point of maximum temperaturewill occur near the distal end of the drive circuit. The heat generatedcauses the first drive beam portion 320 and the second drive beamportion 420 to expand, which bends the first segment 300 in the negativex-direction 325 about the anchor point 360, and bends the second segment400 in the positive y-direction 425 about the substantiallyninety-degree joint 460.

When the current pulse ceases, the drive beam begins to cool. Since thedominant heat sink is located at the contacts 330 and 340, the firstdrive beam portion 320, located closer to the heat sink 330 and 340,cools faster than the second drive beam portion 420, which is locatedfurther from heat sinks 330 and 340. As a result, the first segment 300of the MEMS hysteretic thermal actuator 500 relaxes before the secondsegment 400. Therefore, when the MEMS hysteretic thermal actuator 500 isheating, it bends in a trajectory that is different from the trajectoryupon cooling, resulting in hysteretic behavior when the trajectory isplotted on a graph, as described below.

FIG. 4 is a plot showing the results of a mathematical simulation usingan ANSYS multi-physics finite element model, which simulates thedeflection of the tip of the passive beam that results from the heatingof the drive beam with a square wave current pulse. The current pulseused for FIG. 4 is 190 mA amplitude and 3 μsec duration. Each point inthe plot corresponds to an equal increment of time. As shown in FIG. 4,the tip of the MEMS hysteretic thermal actuator 500 moves in thepositive y-direction and the negative x-direction (the x- and y-axes areindicated in FIG. 3). The movement in the positive y-direction isaccomplished largely by beam portion 400, and movement in the negativex-direction is accomplished largely by beam portion 300. The slope ofthe trajectory of the tip end is approximately −6, such that for everydisplacement of −1 μm in the negative x-direction, the y-displacementincreases by about 6 μm. At the upper left of the displacementtrajectory, at the point labeled A, the current pulse ceases, and thedrive beam begins to cool. The cooling, as described above, relaxes thebeam portion 300 first in the x-direction, followed by the beam portion400 in the y-direction, so that the trajectory of the beams upon coolingis different than the trajectory of the beams upon heating. This isillustrated by the hysteresis seen in the curves shown in FIG. 4. Thehysteresis is evident in the different slope of the upper trajectorycompared to the lower trajectory. The slope of the upper trajectory isabout 5.6 compared to the slope of about 6 for the lower trajectory. Thenominal difference in the location of the tip end on the uppertrajectory compared to the lower trajectory is on the order of about 5μm for this current waveform. This hysteresis may be used to latch andunlatch the MEMS hysteretic actuator, when the actuator is used in aswitch for example, as described further below.

Returning to FIG. 3, MEMS hysteretic thermal actuator 500 may be used toopen and close an electrical switch. To implement this switch, MEMShysteretic thermal actuator 500 is formed with a contact 470, which isadjacent to another contact 480 which is rigidly affixed to thesubstrate. The two contacts 470 and 480 may be made of differentmaterial than segmented beams 310, 320, 410 and 420. The contacts 470and 480 may be made of a material which has a low contact resistancerelative to the material of segmented beams 310, 320, 410 and 420. Inone embodiment, the contacts 470 and 480 are gold, however, othermaterials such as gold-cobalt alloy, palladium, etc., may be used aswell. In this embodiment, an electrical signal may flow from segmentedbeams 310 and 410 to contact 470 and then to contact 480 when the switchis closed. However, in another alternative embodiment described belowwith respect to FIGS. 19, 20 a and 20 b, the electrical signal may flowbetween two contacts located beyond the tip of segmented beam 410,rather than through segmented beam 410 to the contact.

In the quiescent state, the two contacts 470 and 480 of MEMS hystereticthermal switch 500 may be located adjacent to each other, rather thanone in front of the other as is the case with contact flanges 170 and270 shown in FIG. 1.

Because of the location of contacts 470 and 480 may be adjacent to oneanother, contact 470 does not need to be retracted as was shown in FIG.2 a Instead, the sequence of motion for the MEMS hysteretic thermalactuator 500 is shown as shown in FIG. 4, wherein upon energizing thedrive beam, the tip of the cool beam moves up and to the left. Uponcessation of the drive current pulse, the cool beam relaxes on the uppertrajectory shown in FIG. 4, whereupon it becomes engaged on contact 480,because it relaxes more quickly in the x-direction than the y-direction.The spring constant of the MEMS hysteretic thermal actuator 500 causesthe switch to remain latched, because it exerts a normal force on thecontact surfaces 470 and 480. The contact surfaces 470 and 480 remainengaged because of friction between the contact surfaces 470 and 480.Alternatively, the contact surfaces 470 and 480 may be shaped so thatthey remain engaged even without friction. Techniques and designconsiderations for such a switch are described in U.S. patentapplication Ser. No. 11/263,912, which is incorporated by reference inits entirety for all purposes.

To unlatch the MEMS hysteretic actuator 500, a square wave current pulsemay again be applied to the drive beams 320 and 420. The unlatchingcurrent pulse may be of a lower amplitude and/or shorter duration thanthe latching current pulse. The resulting movement of the MEMShysteretic thermal actuator releases the MEMS hysteretic thermalactuator from its engagement with contact 480. The restoring force ofbeam portion 400 may be designed to provide sufficient retraction ofbeam portion 400 to clear the engaging contact 480. The unlatch pulsemay also be tailored in pulse shape, magnitude and duration to assurethat MEMS hysteretic actuator 500 is released from the latched position.

The hysteresis shown in FIG. 4 may also be enhanced, if needed, bytailoring the shape of the current pulse applied to drive beams 320 and420. For example, if the first 3 μsec, 190 mA current pulse is followedimmediately by a second, lower current pulse of 160 mA for another 3μsec, the trajectory of the tip of the MEMS hysteretic actuator 500 isas shown in FIG. 5. In FIG. 5, the second current pulse is applied atthe point labeled B in the graph. The current ceases at the pointlabeled C in the graph, and the upper trajectory describes therelaxation of the MEMS hysteretic actuator 500. The result of the secondcurrent pulse is to hold the MEMS hysteretic thermal actuator 500 inapproximately its deformed shape, while the additional heat provided bythe additional current moves the point of maximum temperature from alocation about ⅔ down the length of the MEMS hysteretic thermal actuator500 to the tip end of the MEMS hysteretic thermal actuator 500. As aresult, the hysteresis experienced by the cooling MEMS hystereticthermal actuator 500 may be exaggerated, because the heat built up inthe tip end of the MEMS hysteretic thermal actuator 500 takes longer todissipate through the far-removed contacts 330 and 340.

Although FIG. 5 shows results for one particular example of a tailoredpulse shape, it should be clear that a large number of alternative pulseshapes or pulse trains can be envisioned, such as a triangular, rampedor saw-toothed pulse shape, to accomplish other objectives with the MEMShysteretic actuator 500, or enhance its performance in other ways.

Additional features of the MEMS hysteretic actuator 500 may be used toadjust the deflection of the MEMS hysteretic actuator 500. For example,areas in the passive beams 310 and 410 may be removed to form a flexiblehinge, to enhance the deflection of the passive beams 310 and 410 abouttheir respective anchor points. Design considerations and implementationof such features are described further in the incorporated '912application.

FIG. 6 is a diagram illustrating a second exemplary embodiment of theMEMS hysteretic thermal actuator 800. In the second exemplaryembodiment, as in the first exemplary embodiment, the MEMS hystereticthermal actuator 800 includes two beam portions 600 and 700 coupled by asubstantially ninety-degree joint 760. Beam portions 600 and 700 arecoupled to pivot anchor 660 and joint 760, respectively. Each beamportion 600 and 700 includes a drive beam portion 620 and 720 and a coolbeam portion 610 and 710. However, in the second exemplary embodiment,the drive beam portions 620 and 720 are disposed on the opposite side ofpassive beam portions 610 and 710, compared to the first exemplaryembodiment. For this reason, MEMS hysteretic thermal actuator 800 bendsin an opposite sense to MEMS hysteretic thermal actuator 500, as drivebeam portion 620 tends to bend passive beam portion 610 in the positivex-direction 625 rather than the negative x-direction. Similarly, drivebeam portion 720 tends to bend passive beam portion 710 in the negativey-direction 725 rather than the positive y-direction. Upon cooling,because of its proximity to the heat sink of contacts 630 and 640, thedrive beam 620 cools more rapidly than drive beam 720, resulting inhysteretic behavior of the MEMS hysteretic actuator 800. Therefore, thebehavior of this MEMS hysteretic actuator 800, if plotted on a graphsimilar to FIGS. 4 and 5, would show the inverse behavior. The tip endof the MEMS hysteretic thermal actuator would therefore be driven to thelower right of FIG. 6. Accordingly, to make an electrical switch usingMEMS hysteretic thermal actuator 800, the contacts 770 and 780 would beplaced as shown in FIG. 6.

FIG. 7 is a diagram illustrating a third exemplary embodiment of theMEMS hysteretic actuator 1100. As with the previous embodiments, theMEMS hysteretic thermal actuator 1100 includes two beam portions 900 and1000 coupled by a substantially ninety-degree joint 1060. Each beamportion 900 and 1000 includes a drive beam portion 920 and 1020 and apassive beam portion 910 and 1010. Drive beam portions 920 and 1020 maybe coupled to passive beam portions 910 and 1010 by tethers 950 and1050, respectively. Tethers 950 and 1050 may be thermally insulating,though not necessarily electrically insulating.

In the second exemplary embodiment, the drive beam portions 920 and 1020are disposed adjacent to a heater element 930, which supplies heat tothe drive beam portions 920 and 1020. The heater element also has a heatsink 940 disposed at its base, which dissipates heat when the heaterelement 930 is disabled. The heater element 930 may include, forexample, an electrical circuit arranged in a serpentine pattern withinheater element 930. For simplicity of depiction, however, the electricalcircuit is not shown in FIG. 7, and the heater element 930 is shown as asimple outline overlaying drive beam portions 920 and 1020. It should beunderstood, however, that the heater element 930 may generate heat inany of a number of other ways, for example, it may be an opticallyopaque element which absorbs incident light.

Upon becoming heated by the heater element 930, drive beam portions 920and 1020 expand, driving passive beam portions 910 and 1010 indirections 925 and 1025, respectively. Upon cooling, because of itsproximity to the heat sink 940 of heater element 930, the drive beam 920cools more rapidly than drive beam 1020, resulting in hystereticbehavior of the MEMS hysteretic actuator 1100. Accordingly, the behaviorof MEMS hysteretic thermal actuator 1100 is similar to that of MEMShysteretic thermal actuator 500, and can be described qualitatively bythe plots shown in FIGS. 4 and 5. A latching electrical switch may bemade using MEMS hysteretic actuator 1100, by disposing contacts 1070 and1080 as shown in FIG. 7.

FIG. 8 is a diagram illustrating a fourth exemplary embodiment of MEMShysteretic actuator 1400. As with the previous embodiments, the MEMShysteretic thermal actuator 1400 includes two beam portions 1200 and1300 coupled by a substantially ninety-degree joint 1360. Each beamportion 1200 and 1300 includes a drive beam portion 1220 and 1320 and apassive beam portion 1210 and 1310. The drive beam portions 1220 and1320 are separated from the passive beam portions 1210 and 1310 by adielectric barrier 1230 which extends out toward the end region of thesecond beam portion 1300, but ends before the edge of second beamportion 1300. The flexibility of the two segments 1200 and 1300 tobending about the anchor point 1260 and rigid link 1360, respectively,may be adjusted by removing an area of material 1230 and 1330, neartheir pivot points, which causes segments 1200 and 1300 to pivot moreeasily about these points.

The drive beam portions 1220 and 1320 may be formed of a material havinga higher coefficient of thermal expansion (CTE), relative to passivebeam portions 1210 and 1310, which are formed of a material having alower coefficient of thermal expansion. However, all of beam portions1220, 1320, 1210 and 1310 are electrically conductive. A current isdriven through drive beam portions 1220 and 1320 to the end of thesecond beam portion 1300, whereupon the current reverses direction andflows out through passive beam portion 1310 and 1210. The current causesjoule heating in both beam portions 1200 and 1300. However, becausedrive beam portions 1220 and 1320 are formed from a material having ahigher coefficient of thermal expansion relative to passive beamportions 1210 and 1310, drive beam portions expand relative to passivebeam portions 1210 and 1310. Accordingly, drive beam portions 1220 and1320 bend the passive beam portions 1210 and 1310 about anchor point1260 and substantially ninety-degree joint 1360, respectively. Uponcooling, because of its proximity to the heat sink of anchor point 1260,the drive beam 1220 cools more rapidly than drive beam 1320, resultingin hysteretic behavior of the MEMS hysteretic actuator 1400.Accordingly, the behavior of MEMS hysteretic thermal actuator 1400 issimilar to that of MEMS hysteretic thermal actuator 500, and can bedescribed by plots similar to those shown in FIGS. 4 and 5. A latchingelectrical switch may be made using MEMS hysteretic actuator 1400, bydisposing contacts 1370 and 1380 as shown in FIG. 8.

FIG. 9 is a diagram illustrating a fifth exemplary embodiment of MEMShysteretic thermal actuator 1700. As with the previous embodiments, theMEMS hysteretic thermal actuator 1700 includes two beam portions 1500and 1600 coupled by a substantially ninety-degree joint 1660. Each beamportion 1500 and 1600 includes a drive beam portion 1520 and 1620 and apassive beam portion 1510 and 1610. The fifth exemplary embodiment isalso similar to the fourth exemplary embodiment, in that the drive beamportions 1520 and 1620 are formed from a material having a highercoefficient of thermal expansion, and the passive beam portions 1510 and1610 are formed from a material having a lower coefficient of thermalexpansion. However, in the fifth exemplary embodiment, beam portions1510, 1520, 1610 and 1620 need not be electrically conductive, becauseheat is supplied by a heater element 1530. Heater element 1530 may be aconductive circuit with wires formed in a serpentine pattern, or may beany other device capable of generating heat. The heat generated byheater element 1530 is absorbed by drive beam portions 1520 and 1620, aswell as passive beam portions 1520 and 1610. However, because drive beamportions 1520 and 1620 are formed from a material having a highercoefficient of thermal expansion relative to passive beam portions 1510and 1610, drive beam portions expand relative to passive beam portions1510 and 1610. Accordingly, drive beam portions 1520 and 1620 bend thepassive beam portions 1510 and 1610 about anchor point 1560 andsubstantially ninety-degree joint 1660, respectively. Upon cooling,because of its proximity to the heat sink of the anchor point 1560, thedrive beam 1520 cools more rapidly than drive beam 1620, resulting inhysteretic behavior of the MEMS hysteretic actuator 1700. Accordingly,the behavior of MEMS hysteretic thermal actuator 1700 is similar to thatof MEMS hysteretic thermal actuator 500, and can be described by plotssimilar to those shown in FIGS. 4 and 5. A latching electrical switchmay be made using MEMS hysteretic actuator 1700, by disposing contacts1670 and 1680 as shown in FIG. 9.

FIG. 10 is a diagram illustrating a sixth exemplary embodiment of theMEMS hysteretic thermal actuator 2000. Like the previous embodiments,MEMS hysteretic thermal actuator 2000 includes two beam portions 1800and 1900 coupled by a substantially ninety-degree joint 1960. Each beamportion 1800 and 1900 includes a drive beam portion 1820 and 1920 and apassive beam portion 1810 and 1910, which are coupled by tethers 1850and 1950, respectively. The drive beam portions 1820 and 1920 are formedfrom stiff, thermally conductive materials. Drive beam portion 1820 isin thermal communication with a circuit 1805, which generates heat atthe base of the drive beam portion 1820. The heat generated by circuit1805 is conducted by thermally conductive drive beam portion 1820 tothermally conductive drive beam portion 1920, causing drive beamportions 1820 and 1920 to heat up. Accordingly, the drive beam portions1820 and 1920 are required to be thermally conductive, but may not beelectrically conductive.

The heating of drive beam members 1820 and 1920 causes drive beamportions 1820 and 1920 to expand. The expansion of drive beam portion1820 causes driven beam 1810 to bend about anchor point 1860 in thenegative x-direction 1825. Similarly, the expansion of drive beamportion 1920 causes driven beam portion 1910 to bend about substantiallyninety-degree joint 1960 in the positive y-direction 1925. Upon cooling,because of its proximity to the heat sink of electrical circuit 1805,the drive beam 1820 cools more rapidly than drive beam 1920, resultingin hysteretic behavior of the MEMS hysteretic actuator 2000.Accordingly, the behavior of MEMS hysteretic thermal actuator 2000 maybe similar to that of MEMS hysteretic thermal actuator 500, and may bedescribed by plots similar to those shown in FIGS. 4 and 5. A latchingelectrical switch may be made using MEMS hysteretic actuator 2000, bydisposing contacts 1970 and 1980 as shown in FIG. 10.

FIG. 11 is a diagram of a seventh exemplary embodiment of a MEMShysteretic thermal actuator 2300. Like the previous embodiments, MEMShysteretic thermal actuator 2300 includes two beam portions 2100 and2200 coupled by a rigid link 2260. However, in this embodiment, therigid link 2260 does not join the two beam portions 2100 and 2200 at asubstantially ninety-degree angle. Instead, rigid link 2260 joins beamportion 2100 and 2100 in a rectilinear fashion. Rigid link 2260 providesa distinct pivot point for beam portion 2200 compared to beam portion2100, which may pivot about anchor point 2160. Accordingly, the presenceof rigid link 2260 allows MEMS hysteretic actuator 2300 to move in twosubstantially different directions, with at least about a five degreeangle between these directions. Each beam portion 2100 and 2200 includesa drive beam portion 2120 and 2220 and a passive beam portion 2110 and2210, which are coupled by tethers 2150 and 2250, respectively.

Heat is generated in drive beam portions 2120 and 2220 by applying avoltage between contacts 2130 and 2140. Current flows in drive beamportions 2120 and 2220 as a result of the voltage, which heats drivebeam portions 2120 and 2220 by joule heating. Drive beam portions 2120and 2220 expand because of their increased temperature. Because drivebeam portions 2120 and 2220 are tethered to passive beam portions 2110and 2210 by tethers 2150 and 2250, the expansion causes passive beamportion 2110 to bend about anchor point 2160, and passive beam portion2210 to bend about rigid link 2260. Upon cooling, the drive beam portion2120 cools faster than drive beam portion 2220, because of its closerproximity to the heat sink of contacts 2130 and 2140. As a result, themotion of MEMS hysteretic thermal actuator 2300 is hysteretic, as thethermal profile of the MEMS hysteretic thermal actuator 2300 isdifferent upon heating than it is upon cooling. By disposing contacts inthe appropriate locations on MEMS hysteretic thermal actuator 2300, alatching electrical switch may be formed.

Although embodiments have been described wherein the first segment isjoined to the second segment at an angle of about zero degrees (FIG. 11)and an angle of about ninety degrees (FIGS. 3 and 6-10), it should beunderstood that any other angle greater than or equal to about zerodegrees and less than or equal to about ninety degrees may also be usedin the MEMS hysteretic actuator.

FIG. 12 is a schematic side view of an eighth exemplary embodiment of aMEMS hysteretic thermal actuator 2600. Like the previous embodiments,MEMS hysteretic thermal actuator 2600 includes two beam portions 2400and 2500 coupled by a rigid link 2560. Each beam portion 2400 and 2500includes a drive beam portion 2420 and 2520 and a passive beam portion2410 and 2510, which are coupled by tethers 2450 and 2550, respectively.However, in this embodiment, drive beam portions 2420 and 2520 aredisposed such that they drive the driven beam portions 2410 and 2510 intwo different planes. In particular, drive beam portion 2420 is orientedto bend passive beam portion 2410 about fixed anchor point 2460 indirection 2425, indicated in FIG. 12. Drive beam portion 2520 isoriented to bend passive beam portion 2510 about the rigid link 2560 inthe direction 2525, which is into the paper as indicated in FIG. 12.Accordingly, MEMS hysteretic thermal actuator has one component bendingin the plane of the paper, and another component bending in a planeorthogonal to the paper. Like the previous embodiments, current is inputto drive beam portions 2420 and 2520 by applying a voltage betweencontacts 2430 and 2440. The current heats the drive beams by jouleheating, and the resulting expansion of the drive beam portions 2420 and2520 causes the bending of the passive beams 2410 and 2510 describedabove. The contacts 2430 and 2440 also provide a heat sink for the drivebeam portions 2420 and 2520. Accordingly, drive beam portion 2420located nearer to heat sink contacts 2430 and 2440 cools more quicklythan drive beam portion 2520 located farther from heat sink contacts2430 and 2440. Therefore, the motion of MEMS hysteretic thermal actuator2600, like MEMS hysteretic thermal actuators 500-2300 is hystereticbetween the heating phase and the cooling phase. FIG. 13 is aperspective view of the eighth exemplary embodiment described above.

One of the issues with the eighth exemplary embodiment of MEMShysteretic thermal actuator 2600 is difficulty of manufacturing. Asshown in FIG. 13, in the first segment 2400 of MEMS hysteretic thermalactuator 2600, the cantilevered drive beams 2420 are disposed adjacentto the passive beam portion 2410, whereas in the second portion 2500 ofMEMS hysteretic thermal actuator 2600, the cantilevered drive beams 2420are disposed above the passive beam portion 2510. Because of theorientations of the drive beam portions 2420 and 2520 with respect tothe passive beam portions 2410 and 2510, the drive beam portions cannotbe formed or deposited in a single step. This complicates themanufacturing process flow for making MEMS hysteretic thermal actuator2600.

FIG. 14 illustrates a ninth exemplary MEMS hysteretic thermal actuator3000, which may be easier to manufacture than MEMS hysteretic thermalactuator 2600. MEMS thermal device 3000 is similar to hysteretic MEMSthermal device 2600, in that it is designed to move in two substantiallyorthogonal planes. In hysteretic MEMS thermal device 3000, the drivebeam is separated into two segments, 3200 and 3300, which are separatedby a flexure 3250. The flexure 3250 serves to decouple the motion in thetwo orthogonal planes, which motion is driven by a first segment 3200and a second segment 3300. The first plane of motion is out of the planeof the paper, in direction 3225, as indicated in FIG. 14. This motion isproduced by fabricating the cantilevered drive beam 3200 slightly belowthe plane of the passive beam segment 3500, relative to the substratesurface. That is, cantilevered drive beam 3200 is closer to thesubstrate surface than passive beam segment 3500. More generally, thedriving beam segment 3200 may have an average elevation different thanthe average elevation of the passive beam segment 3500, wherein theaverage elevation is along the longitudinal axis of driving beam 3200.Accordingly, when cantilevered drive beam segment 3200 is heated bypassing a current through cantilevered drive beam segment 3200 byapplying a voltage to pads 3130 and 3140, it expands about the pads 3130and 3140 anchored to the surface of the substrate. Because passive beamsegment 3500 is not heated, it does not expand, so that cantilevereddrive beam segment 3200 is forced to bend itself and passive beamsegment 3500 upward, away from the substrate surface, about the anchorpoint 3530 of the passive beam segment 3500.

Then, the second segment 3300 of cantilevered drive beam begins to heatas the current is passes through it. It also expands as a result of theheat, and begins to move in direction 3325, bending passive beam segment3600 in this direction. Cantilevered drive beam segment 3300 and passivebeam segment 3600 move in direction 3325 because they are formed in thesame plane, so that as cantilevered drive beam segment 3300 heats up, itbends toward the unheated passive beam segment 3600 in direction 3325.Accordingly, driving beam segment 3300 has an average elevationsubstantially the same as passive beam segment 3600.

At the joint between cantilevered drive beam segment 3200 andcantilevered drive beam segment 3300 is flexure 3250. The flexure 3250consists of a length of the hot driving beam which bends away from andthen back toward a knee 3255 in the passive beam, thus adding length tothe hot driving beam. This additional length adds to the heat producedin the hot driving beam, by increasing its resistance. The amount ofheat created within the flexure 3250 is larger than in the driving beamsegments 3200 and 3300 because of the larger distance flexure 3250 isfrom the passive beam segments 3500 and 3600 which may act as radiativeheat sinks. Therefore, the flexure 3250 acts as a heat choke, whichimpedes the flow of heat from the tip of the driving beam segment 3300back to the anchor points 3130 and 3140, which act as the primary heatsink for the device. The presence of the flexure therefore enhances thehysteresis of the device, because heat built up in the tip of thedriving beam segment 3300 has greater difficulty returning to the heatsink anchor point 3130 and 3140 than heat built up in the first drivingbeam segment 3200. It should be understood that additional lengths ofdriving beam may also be added to driving beam segment 3300 to increasethe temperature at the distal end of MEMS hysteretic thermal actuator3000, thus increasing the hysteresis of the actuator 3000.

In addition, flexure 3250 acts as a mechanical component to decouple theout-of-plane motion of the first beam segments 3200 and 3500, from thein-plane motion of the second beam segments 3300 and 3600. This functionis provided primarily by the presence of dielectric spacers 3251 and3253 between the driving beam segment 3200 and the knee 3255 of thepassive segment 3500 and dielectric spacers 3355 between driving beamsegment 3300 and passive segment 3600. Dielectric spacers 3253 act totransmit the out-of-plane torque produced by driving beam segment 3200to passive beam segment 3500 by tethering the segments together at thatpoint to bend the passive segment 3500 in direction 3225. Dielectricspacers 3251 act as an anchor for the bending of driving beam segment3300 in direction 3325. The torque from driving beam segment 3300 istransmitted to the passive segment 3600 by dielectric spacers 3355, tobend the passive beam in direction 3325, at hinge flexure 3650.

In one exemplary embodiment, flexure 3250 may have a width a of about 48μm and a height b of about 32 μm. The length of driving beam segment3200 may be 270 μm, with a beam segment width of about 5 μm. The lengthof driving beam segment 3300 may be about 128 μm, so that the flexure3250 is located about ⅔ of the distance between the anchor points 3130and 3140 and the tip of driving beam segment 3300. However, it should beunderstood that these dimensions are exemplary only, and that othershapes and dimensions may be chosen depending on the requirements of theapplication. For example, the flexure 3250 may alternatively be locatedat about ⅓ or ½ of the distance between the anchor points 3130 and 3140and the tip of the driving beam segment 3300.

FIG. 15 shows the ninth exemplary embodiment in perspective view, inwhich the construction of the ninth embodiment is more clearly evident.The first driving beam segment 3200 is fabricated on a lower plane thanpassive beam segment 3500. This allows the driving beam segment 3200 tobend the passive beam segment 3500 up and out of plane. In contrast,driving beam segment 3300 is in the same plane as passive segment 3600,in order to bend passive segment 3600 in the same plane as driving beamsegment 3300. Thus, a step up occurs in the driving beam between drivingbeam segments 3200 and 3300. This step up may occur adjacent todielectric tethers 3253, as the driving beam segment 3200 enters flexureregion 3250. Accordingly, passive beam segments 3500 and 3600 may all bein the same plane, allowing the cool, passive beam segments 3500 and3600 to be formed in a single process step, and simplifyingmanufacturing. Similarly, the driving beam segments 3200 and 3300 areall formed on the same side of passive beam segments 3500 and 3600,allowing the driving beams segments to be formed in a single depositionstep, as described below.

Some additional features shown in FIG. 15 give the ninth exemplaryembodiment several advantageous characteristics. First, passive beamsegment 3600 has a relieved, hinged area 3650, which gives it additionalflexibility to bend in direction 3325. Secondly, passive beam segment3500 has a rectangular box shape. This gives passive beam segment 3500 ahigh moment of inertia in the in-plane direction but a low moment ofinertia in the out of plane direction. This configuration enhances theability of MEMS hysteretic thermal actuator 3000 to move out of planerather than in-plane.

Additional structures, such as heat sink 3550 may be added to passivebeam segment 3500 to assist with transferring heat through conduction,convection, and radiation away from passive beam segment 3500. Thetemperature difference between driving beam segment 3200 and passivebeam segment 3500 may be proportional to the magnitude of out of planemovement that can be achieved. Heat may be transferred to passive beamsegment 3500 from driving beam segment 3200 and the insulators 3253.Passive beam segment 3500 does not have a good thermal conduction pathto anchor 3530 due to its thin width required to lower the out of planestiffness. Additional heat sink 3550 may be added to increase the areaof passive beam segment 3500 that can transmit heat without changing thestiffness. If more in-plane stiffness is required, heat sink 3550 can bedesigned to triangulate the box shape and increase the in-planestiffness significantly more than the out of plane stiffness. Thesefeatures thus enhance the ability of MEMS hysteretic thermal actuator3000 to move out of plane rather than in-plane. Thus, the out-of-planemotion of passive beam segment 3500 is effectively decoupled fromin-plane motion of passive beam segment 3600 by structures 3550 and3650, as well as by flexure 3250.

The hysteretic effect in the ninth exemplary embodiment of MEMShysteretic thermal actuator 3000 arises from the same effect as for MEMShysteretic thermal actuators 500-2600. The hysteresis arises from theproximity of the primary heat sink to one of the segments of MEMShysteretic thermal actuator 3000. This cools the segment nearest theheat sink faster than the segment further away from the heat sink,whereas during heating, the two segments are heated relativelyuniformly. In the case of MEMS hysteretic thermal actuator 3000, bothsegments 3200 and 3300 are heated relatively uniformly, moving the tip3600 diagonally away from the surface of the paper, up from the surfaceof the paper because of driving beam segment 3200 and downward indirection 3325 because of the action of driving beam segment 3300.However, upon cooling, driving beam segment 3200 cools faster thandriving beam segment 3300, because of its closer proximity to heatsinking anchor points 3130 and 3140. MEMS hysteretic thermal actuator3000 may therefore move back toward the substrate surface beforerelaxing back in the upward direction, opposite to direction 3325.Accordingly, because MEMS hysteretic thermal actuator moves throughdifferent points upon heating as it does upon cooling, MEMS hystereticthermal actuator has a different trajectory upon activation, for exampleupon heating, than it does upon relaxation, for example upon cooling.For this reason, MEMS hysteretic thermal actuator 3000 may be used torise up and over a stationary contact 3700, landing on the contact atthe end of a latching pulse and remaining there because of frictionalforces, or by forming a détente structure on the contact. The MEMShysteretic thermal actuator 3000 may then be unlatched by applying acurrent pulse of appropriate amplitude and duration.

The ability of MEMS hysteretic thermal actuator 3000 to rise up and overa stationary contact 3700, is illustrated in FIGS. 16-19. FIG. 16depicts MEMS hysteretic thermal actuator 3000 in its initial position,adjacent to a stationary contact 3700. By applying a voltage to anchorpoints 3130 and 3140, a current flows through driving beam segments 3200and 3300, heating them by Joule heating. This causes driving beamsegment 3200 to expand relative to passive segment 3500, such thatdriving beam segment 3200 bends passive segment 3500 about its anchorpoint 3530, and away from the plane of the paper. At the same time,driving beam segment 3300 also heats and expands, bending passivesegment 3600 about its pivot point at hinged flexure 3650. This causesMEMS hysteretic thermal actuator 3000 to rise up and over stationarycontact 3700, as shown in FIG. 17.

When the voltage applied to anchor points 3130 and 3140 is removed,current ceases to flow and heat ceases to be generated. At this point,the heat flows back out of the device mainly through anchor points 3130and 3140. This cools driving beam segment 3200 before driving beamsegment 3300 cools, which lowers MEMS hysteretic thermal actuator 3000onto the stationary contact 3700. This situation is depicted in FIG. 18.MEMS hysteretic thermal device 3000 may remain latched on stationarycontact by frictional forces or a combination of frictional and normalforces. This latching may close, for example, an electrical switch.

Using a latching current pulse of about 3000 μsec in duration and about160 mA in amplitude, MEMS hysteretic thermal actuator 3000 may beexpected to move a total diagonal distance of about 11 μm, rising byabout 8 μm and moving laterally about 8 μm.

When it is desired to open the electrical switch, a shorter durationand/or lower amplitude pulse may be applied to MEMS hysteretic thermalactuator 3000, which will raise the driving beam segment 3200sufficiently to free the tip of passive segment 3600 from the stationarycontact, as described earlier with respect to MEMS hysteretic thermalactuator 500. The unlatching is shown in FIG. 19. MEMS hystereticthermal actuator 3000 will then immediately move back in direction 3326.In one exemplary embodiment, the unlatching pulse has a duration of 1000μsec and 160 mA.

Although the embodiments 500-3000 described above each have at least twosubstantially straight beam segments in the cantilever, it should beunderstood that a MEMS hysteretic device may also be formed using asingle cantilevered arcuate beam. In this embodiment, the arcuateactuator is disposed adjacent to an arcuate drive beam, and tethered tothe drive beam by at least two dielectric tethers, one at the tip of thearcuate actuator and one at an intermediate location. The amount ofhysteresis provided by such an arcuate embodiment may depend on thecurvature of the cantilever and the location of the dielectric tethers.However, in general, the cantilever having two segments disposedorthogonally to each other may have a larger amount of hysteresis, andmay therefore be more suitable for making a latching switch.

An exemplary method for fabricating the MEMS hysteretic actuator 3000will be described next. Although the method is directed primarily to thefabrication of MEMS hysteretic actuator 3000, it should be understoodthat the method may be adapted to fabricate MEMS hysteretic actuators500-2600. The MEMS hysteretic actuator 3000 may be fabricated on anyconvenient substrate 4000, for example silicon, silicon-on-insulator(SOI), glass, or the like.

Because in FIGS. 20-27, the MEMS hysteretic actuator is shown in crosssection, only a few of the beam segments of the MEMS hysteretic thermalactuator can be seen in the figures, as the other segments may bedisposed in a plane which is not easily depicted in the figures. Forexample, in FIG. 27, element 4200 may be understood to depict drivingbeam segment 3200 in MEMS hysteretic actuator 3000. Element 4300,corresponding to driving beam segment 3300, lies partially in the sameplane as element 4200, and behind element 4200, and is thus partiallyobscured by element 4200. However, it should be understood that thedriving beam segment 3300 may be formed at the same time as, and usingsimilar or identical processes to those used to form the driving beamsegment 4200 which is depicted in FIGS. 20-27. Similarly, only onestructure 4500, may represent passive segment 3500 of the MEMShysteretic thermal actuator 3000 in FIGS. 20-27. However, it should beunderstood that all components of passive beam 3500, for example heatsink 3550 may be formed at the same time, using similar or identicalprocess steps used to form structure 4500. Accordingly, both the seconddriving beam portion 3300, the heat sink 3550 and second passive segment3600 of MEMS hysteretic thermal actuator 3000 may be formed at the sametime, using the same process steps and materials, as used to formsegments 4200, 4300 and 4500, representing driving beam segment 3200,3300 and passive segment 3500, respectively.

FIG. 20 illustrates a first exemplary step in the fabrication of theMEMS hysteretic thermal actuator 3000. The process begins with thedeposition of a seed layer 4010 over the substrate 4000. The seed layer4010 may be chromium (Cr) and gold (Au), deposited by plasma vapordeposition (PVD) to a thickness of 100-200 nm. Photoresist (not shown)may then be deposited over the seed layer 4010, and patterned byexposure through a mask. A sacrificial layer 4020, such as copper, of athickness of 2 μm may then be electroplated over the seed layer throughthe photoresist, as depicted in FIG. 21. The plating solution may be anystandard commercially available or in-house formulated copper platingbath. Plating conditions may be particular to the manufacturer'sguidelines. However, any other sacrificial material that can beelectroplated may also be used. In addition, deposition processes otherthan plating may be used to form sacrificial layer 4020. The photoresistmay then be stripped from the substrate 4000.

FIG. 22 illustrates a second step in an exemplary process forfabricating MEMS hysteretic thermal actuator 3000. In the second step,another sacrificial layer 4030 is formed over the substrate 4000 and thefirst sacrificial layer 4020. The second sacrificial layer 4030 may alsobe copper, with a thickness of about 5 μm. The purpose of the twosacrificial layers 4020 and 4030 is to provide two levels which allowthe offset between the elevations of driving beam segment 3200 andpassive segment 3500, which provides the out-of-plane movement of thefirst segment of MEMS hysteretic thermal actuator. The dashed line inFIG. 22 indicates areas further behind the indicated cross section whichrequire a higher elevation, such as for driving beam segment 3300 inFIGS. 14-16. The second sacrificial layer 4030 is also deposited inthese areas to provide this higher elevation. This step second step maybe performed in the same way as for the first sacrificial layer 4020, bydepositing and patterning photoresist, plating the sacrificial layer4030 on the first sacrificial layer through the photoresist, andstripping the photoresist.

A third step in the exemplary method for fabricating the MEMS hystereticthermal actuator 3000 is illustrated in FIG. 23. In FIG. 23, thesubstrate 4000 is again covered with photoresist, which is exposedthrough a mask with features corresponding to stationary contact 3700and a corresponding second contact at the distal end of the passivesegment 3600. For simplicity, this second contact was not shown in FIGS.14-19. A conductive material with good electrical transport propertiesand good contact resistance may then be plated in areas 4600 and 4700.The additional contact material 4600 may be used at the distal end ofpassive segment 3600 because it may have lower contact resistance thanthe material that will form the beam 3600. In one exemplary embodiment,gold (Au) is chosen as the contact material 4600 and 4700. Additionalgold features may also be plated in this step, such as a bonding ring,which may eventually form a portion of a hermetic seal which may bond acap wafer over the substrate 4000 and MEMS hysteretic thermal actuator3000. Another of the gold features may be an external access pad thatwill provide access to the MEMS hysteretic actuator electrically, fromoutside the hermetically sealed structure.

Gold may then be electroplated in the areas not covered by thephotoresist, to form gold features 4600 and 4700, and any other goldstructures needed. The photoresist is then stripped from the substrate4000. The thickness of the gold features 4600 and 4700 may be, forexample, 5 μm. Because the gold contact features 4600 and 4700 may beplated over both the first sacrificial layer 4020 and the secondsacrificial layer 4030, the gold contacts may be formed with a lip 4650at a lower elevation than the rest of gold contact feature 4600. Thislip 4650 may mate with a corresponding lip on the stationary goldcontact 4700, to form a détente position for the closed switch, so thatthe switch may be positively latched rather than relying on frictionalforces to keep the switch closed. However, it should be understood thatMEMS hysteretic thermal actuator 3000 may be formed without this lip aswell, as shown in FIGS. 14-19.

If desired or needed, an optional step may be included in the process atthis point, which is illustrated in FIG. 24. This optional step is thedeposition of another contact material over the surface of thestationary gold contact 3700 or 4700. This additional contact materialmay be chosen to improve the mechanical properties of the contactmetals. For example, palladium, or a gold-cobalt alloy may be sputteredover stationary contact 4700, which may reduce the tendency of the twogold contacts 4600 and 4700 to adhere to one another after touching.Other additional metals may include ruthenium, platinum, gold-platinumalloy, and gold-nickel alloy, for example.

FIG. 25 illustrates a fifth step in an exemplary method for fabricatingthe MEMS hysteretic thermal actuator 3000. In FIG. 25, photoresist (notshown) is once again applied over the substrate 4000, and patternedaccording to the features in a mask. The exposed portions of thephotoresist are then dissolved as before, exposing the appropriate areasof the seed layer 4010 and sacrificial layers 4020 and 4030. The exposedseed layer 4010 and sacrificial layers 4020 and 4030 may then beelectroplated with nickel to form the features 4200, 4300 and 4500. Asdescribed above, feature 4200 may correspond to driving beam segment3200, and feature 4500 may correspond to passive segment 3500. Feature4300 may correspond to driving beam 3300 in FIGS. 14-16, and isdeposited over the second sacrificial layer 4030, shown as the dashedline in FIG. 22. Although only beam segments 3200, 3300 and 3500 areshown in this step, it should be understood that passive beam segment3600 may be formed using substantially similar or identical techniques,at the same time as passive beams segment 3200, 3300 and 3500. However,since passive beam segment 3600 may be located directly adjacent to, andin the same plane as driving beam portion 3300 and passive segment 3500,only driving beam segments 3200, 3300 and passive segments 3500 aredepicted in FIG. 25. Furthermore, although passive beam 3500 isrepresented only by the single feature 4500, it should be understoodthat any additional structures such as heat sink 3550 may be formedusing similar or identical processes to those shown. The gold contact4600 may be affixed to the nickel beam 4500 by the natural adhesion ofthe gold to the nickel, after deposition. Although nickel is chosen inthis example, it should be understood that any other conductive materialthat can be electroplated may also be used. In addition, depositionprocesses other than plating may be used to form beams 4200 and 4500.The photoresist may then be stripped from the substrate 4000.

FIG. 26 illustrates a fifth step in the fabrication of the MEMShysteretic thermal actuator 3000. In FIG. 17, a polymeric, nonconductingmaterial 4800 such as the negative tone photo-patternable polymer,photoresist SU-8 is deposited over the substrate 4000, and beams 4200and 4500. The photoresist 4800 is then cross-linked, by for example,exposure to UV light. The unexposed resist is then dissolved and removedfrom the substrate 4000 and structures 4200 and 4500 in all areas thatthe dielectric tether should be absent. This step may form thedielectric tethers 3251 and 3253, as well as other tethers indicated inthe preceding FIGS. 14-19. The remaining photoresist may then be curedby, for example, baking. While SU-8 is used in this embodiment, itshould be understood that this is exemplary only, and that any othernon-conducting material may be used to form the tethers 4800.

FIG. 27 illustrates a sixth step in the fabrication of the MEMShysteretic thermal actuator 3000. In this step, the beams 4200, 4500 and4600 may be released by etching the underlying sacrificial copper layers4020 and 4030. Suitable etchants may include, for example, an isotropicetch using an ammonia-based Cu etchant. The Cr and Au seed layer 4010may then also be etched using, for example, a wet etchant such asiodine/iodide for the Au and permanganate for the Cr, to expose the SiO₂surface of the substrate 4000. Although not shown in FIG. 27, it shouldbe understood that structures 4200 and 4500 may be anchored to thesubstrate 4000 by anchors plated in an area without sacrificial layers4010 and 4020. These anchors may form anchor points 3130, 3140 and 3530.However, because the cross section shown does not include any anchorpoints, they are not shown in FIG. 27. The substrate 4000 with the MEMShysteretic thermal actuator 3000 may then be rinsed and dried, and itsfabrication is essentially complete.

The resulting MEMS hysteretic actuator 3000 may then be encapsulated ina protective lid or cap wafer. Details relating to the fabrication of acap wafer may be found in co-pending U.S. patent application Ser. No.11/211,625, incorporated by reference herein in its entirety.

It should be understood that one gold feature may be used as an externalaccess pad for electrical access to the MEMS hysteretic thermal actuator3000, such as to supply a signal to the MEMS hysteretic thermal actuator3000, or to supply a voltage the terminals 3130 or 3140 in order toenergize the drive loops of the actuator, for example. The externalaccess pad may be located outside the bond line which will be formedupon the bonding of a cap layer to the substrate 4000. Alternatively,electrical connections to MEMS hysteretic actuator may be made usingthrough-wafer vias, such as those disclosed in co-pending U.S. patentapplication Ser. No. 11/211,624, U.S. patent application Ser. No.11/482,944, and U.S. patent application Ser. No. 11/541,774, each ofwhich is incorporated herein by reference in its entirety.

In each of the previous embodiments, an electrical signal is presumed toflow along the cantilevered beam to a stationary contact located underthe tip of the beam. However, it is also envisioned to configure theMEMS hysteretic device such that an electrical signal flows between twostationary electrodes located on the substrate surface under the MEMShysteretic actuator itself. In this case, the contact material 4600disposed on the distal end of the second beam segment may provide theelectrical connection between the stationary electrodes. Such anexemplary embodiment is described in co-pending U.S. patent applicationSer. No. 11/334,438, incorporated by reference herein in its entirety.

While various details have been described in conjunction with theexemplary implementations outlined above, various alternatives,modifications, variations, improvements, and/or substantial equivalents,whether known or that are or may be presently unforeseen, may becomeapparent upon reviewing the foregoing disclosure. For example, whileMEMS hysteretic thermal actuators are described which have two segments,it should be understood that any number of additional segments may alsobe used. Furthermore, although the cantilevers are described as havingstraight segments, it should be understood that this is exemplary only,and that the cantilever may also have an arcuate shape. While theembodiments described above relate to a microelectromechanical actuator,it should be understood that the techniques and designs described abovemay be applied to any of a number of other microelectromechanicaldevices, such as valves and switches. Accordingly, the exemplaryimplementations set forth above, are intended to be illustrative, notlimiting.

1. A method of manufacturing a hysteretic micromechanical device on asurface of a substrate comprising: forming a first beam segmentconfigured to move in a direction having a component perpendicular tothe substrate surface; forming a second beam segment configured to movein a direction having a component parallel to the substrate surface;forming a flexure joining the first beam segment to the second beamsegment, wherein a distal end of the second beam segment is driven in afirst trajectory during activation and a second trajectory duringrelaxation of the first beam segment and the second beam segment.
 2. Themethod of claim 1, wherein forming the first beam segment and the secondbeam segment further comprises: forming the first beam segment coupledat a proximal end to the substrate at an anchor point, and to theflexure at a distal end; forming the second beam segment coupled at aproximal end to the flexure, and coupled to a contact at the distal end.3. The method of claim 2, wherein forming the first beam segment and thesecond beam segment comprises: forming a driving beam segment whichexpands when current is driven through the driving beam segment; andforming a passive beam segment; coupling to the driving beam segment tothe passive beam segment with at least one dielectric tether, so thatthe driving beam segment transmits motion to the passive beam segment bythe dielectric tether.
 4. The method of claim 3, wherein coupling thedriving beam segment to the passive beam segment comprises: applying aphoto-patternable polymer over the driving beam segment and the passivebeam segment; exposing and developing the photo-patternable polymer toform the dielectric tether between the driving beam segment and thepassive beam segment; and baking the photo-patternable polymer to formthe dielectric tether.
 5. The method of claim 3, further comprising:releasing the first beam segment and the second beam segment by removinga sacrificial layer beneath the first beam segment and the second beamsegment, except in the vicinity of the anchor point.
 6. The method ofclaim 3, wherein the at least one dielectric tether comprises aphoto-patternable polymer.
 7. The method of claim 2, wherein forming thefirst beam segment and the second beam segment comprises: forming thefirst beam segment and the second beam segment by plating at least oneof nickel and a nickel alloy onto a seed layer through a photoresiststencil.
 8. The method of claim 2, further comprising: plating a contactmaterial on a sacrificial layer; forming the second beam segment overthe sacrificial layer such that a distal end of the second beam segmentis coupled to the contact material.
 9. The method of claim 2, whereinthe first beam segment and the second beam segment comprise at least oneof nickel and a nickel alloy.
 10. The method of claim 1, wherein formingthe first beam segment comprises: forming a first driving beam segmentwhich expands when current is driven through the driving beam segment;and forming a first passive beam segment coupled to the first drivingbeam segment and moved by the expansion of the first driving beamsegment.
 11. The method of claim 10, wherein the first driving beamsegment is disposed at an average elevation different from an averageelevation defined by the first passive beam segment.
 12. The method ofclaim 11, wherein the flexure provides a conductive path between thefirst driving beam segment of the first beam segment and the seconddriving beam segment of the second beam segment, and wherein the firstpassive beam segment of the first beam segment has at least one of arectangular box shape and a triangular shape.
 13. The method of claim 1,wherein forming the second beam segment comprises: forming a seconddriving beam segment which expands when current is driven through thedriving beam segment; and forming a second passive beam segment coupledto the second driving beam segment and moved by the expansion of thedriving beam segment.
 14. The method of claim 13, wherein the seconddriving beam portion disposed adjacent to and at substantially the sameaverage elevation as the passive beam portion.
 15. The method of claim13, further comprising: forming at least one stationary electrodeaffixed to the substrate surface, wherein the contact material isconfigured to make contact with the stationary electrode after thecurrent is applied to the first and the second driving beam segments,wherein the stationary electrode comprises at least one of palladium,gold, a gold-cobalt alloy, ruthenium, platinum, gold-platinum alloy, andgold-nickel alloy.
 16. The method of claim 1, further comprising:coupling the first beam segment to the substrate at an anchor pointdisposed at a proximal end of the first beam segment; and coupling thesecond beam segment to the first beam segment at the flexure, whereinthe flexure defines a proximal end of the second beam segment.
 17. Amethod of manufacturing a hysteretic micromechanical device on a surfaceof a substrate comprising: forming a first beam segment coupled to thesubstrate at an anchor point at a proximal end and configured to move ina direction having a component perpendicular to the substrate surface;forming a second beam segment coupled to a contact a distal end, andconfigured to move in a direction having a component parallel to thesubstrate surface; forming a flexure joining the first beam segment at adistal end and a the second beam segment at a proximal end, wherein thefirst beam segment and the second beam segment are driven in a firsttrajectory during activation and a second trajectory during relaxationplating a contact material on a sacrificial layer; forming the secondbeam segment over the sacrificial layer such that a distal end of thesecond beam segment is coupled to the contact material; forming astationary contact on the substrate surface by plating at least one ofgold, palladium, a gold-cobalt alloy, ruthenium, platinum, agold-platinum alloy, and gold-nickel alloy over the substrate surface,wherein the stationary contact interacts with the contact material onthe distal end of the second beam segment to form terminals of anelectrical switch.
 18. The method of claim 17, further comprising:forming the contact material at a distal end of the second beam segment,and wherein the flexure is located at about ⅔ of a distance from theanchor point to the distal end of the second beam segment.
 19. Anapparatus for forming a hysteretic micromechanical device on a surfaceof a substrate, comprising: means for forming a first beam segmentconfigured to move in a direction having a component perpendicular tothe substrate surface; means for forming a second beam segmentconfigured to move in a direction having a component parallel to thesubstrate surface; means for forming a flexure joining the first beamsegment to the second beam segment, wherein a distal end of the secondbeam segment is driven in a first trajectory during activation and asecond trajectory during relaxation of the first beam segment and thesecond beam segment.